Electron beam apparatus comprising a point cathode

ABSTRACT

The electron source of an electron beam apparatus comprises a cathode wire which is to be heated by a laser beam and which is to be displaced in the wire direction. This cathode wire has a thickness of from 10 to 30 microns. By applying a field strength of 105 to 106 KV/m to the heated wire tip and by controlling the laser intensity and the wire feed during operation by a signal derived from the emission of the wire tip, a tip is produced on the wire end having a curvature radius of approximately 1 micron. At a temperature just below the melting temperature of the wire a stable temperature field emission having a current density of up to better than 104 A/cm2 is thus realized.

[451 Feb. 4, 1975 ELECTRON BEAM APPARATUS COMPRISING A POINT CATHODE Inventors: Karel Diederick van der Mast;

James Edmond Barth, both of Delft, Netherlands U.S. Philips Corporation, New York, N.Y.

Filed: Oct. 2, 1973, Appl. 1 10.; 402,844

Assignee:

US. Cl. 250/306, 250/310 Int. Cl. H01j 37/26 Field of Search 250/311, 310, 306, 307,

References Cited UNITED STATES PATENTS Primary ExaminerJames W. Lawrence Assistant Examiner-C. E. Church Attorney, Agent, or Firm-Frank R. Trifari [57] ABSTRACT The electron source of an electron beam apparatus comprises a cathode wire which is to be heated by a laser beam and which is to be displaced in the wire direction. This cathode wire has a thickness of from 10 to 30 microns. By applying a field strength of 10 to 10 KV/m to the'heated wire tip and by controlling the laser intensity and the wire feed during operation bya signal derived from the emission of the wire tip, a tip is produced on the wire end having a curvature radius of approximately 1 micron. At a temperature just below the melting temperature of the wire a stable temperature field emission having a current density of up to better than 10" A/cm is thus realized.

9 Claims, 3 Drawing Figures ELECTRON BEAM APPARATUS COMPRISING A POINT CATHODE The invention relates to an electron beam apparatus provided with an electron source with a cathode wire, one free end of which is displaceable in a beam path of an auxiliary radiation source.

An electron beam apparatus of this kind is known, for example, from German Pat. No. l,028,244. The cathode used therein consists of a tungsten wire having a diameter of from 100 to 200 microns, the end of which is heated by an ion beam. The ion beam is directed onto the wire tip by an electric field which is formed between an ion source having a positive potential with respect to the cathode tip and an electrode which is arranged opposite to the cathode tip and which has a negative potential with respect thereto. The maximum thermal electron emission which can be delivered by a cathode of this kind is determined by the material, the geometry and the temperature of the wire tip. If this cathode is set for a high emission current density, the temperature becomes very high, thus causing a comparatively large evaporation of the cathode tip. Therefore, the cathode wire can each time be advanced after a given period of operation. The evaporated material, however, will substantially contaminate the interior of the apparatus and the intermittent advancing of the wire will cause variations in the emission current.

The invention has for its object to eliminate these drawbacks, and to this end an electron beam apparatus of the kind set forth according to the invention is characterized in that opposite to the free end of the cathode wire a punctured electrode is arranged for generating a positive field strength of approximately 10 to KV/m on the surface of the wire end, a coupling between the energy supply from the auxiliary radiation source to the wire end and the movement of the cathode during operation maintaining a point-like end having an at least substantially constant curvature radius of between about 0.2 microns and 2 microns on the cathode wire at a temperature at which deformations occur due to field strength forces. This curvature radius is substantially larger than in field emission sources. As a result, the loss of effective brightness by lens defects which unavoidably occur in the electron-optical system comprising the source is much smaller than in these sources.

By using, according to the invention, a cathode wire having a thickness of approximately 10 microns to 30 microns and by heating this wire, preferably by means of a laser beam, on one free end and by displacing the cathode wire in the laser beam during operation, a wire tip having a curvature radius of the desired dimension is maintained if the electric field strength on the surface of the wire end is suitably chosen. According to the invention, the transporting of the cathode wire is controlled by a quantity which is at least co-determined by the temperature of the wire tip. The wire tip remains requirements are satisfied for the so-termed temperature-field emission produced by the Schottky effect. Due to this effect, occurring in the case of a sufficiently high field strength on the surface and a sufficiently high temperature of the cathode material, the electrons need neither overcome the entire exit potential (work function), such as is necessary in the case of thermal emission, nor to tunnel the potential barrier as is necessary in the case of field emission. The exit potential to be overcome is substantially reduced by the applied electric field, with the result that at the same temperature the emission can be some factors higher per unit of surface area.

A striking and very favourable side-effect of the operation at a temperature just below the melting point of the cathode material is that the electron emission is thus rendered completely independent of the crystal direction. Because at a less high temperature the emission density is substantially lower in the wire direction than in the directions transverse thereto, an additional gain is obtained for the beam current.

Some preferred embodiments according to the invention will be described in detail hereinafter with reference to the drawing.

FIG. 1 is a diagrammatic representation of an electron beam apparatus according to the invention which is constructed as a scanning electron microscope,

FIG. 2 is a diagrammatic view of a preferred embodiment according to the invention, comprising a laser as controllable auxiliary radiation source, and

FIG. 3 shows a preferred embodiment for transporting and holding the cathode wire according to the invention.

The scanning electron microscope which is diagrammatically shown in FIG. 1 comprises a cathode wire 2 which is mounted in a holder 1, a first anode 3 and a second anode 4. A free end or tip 5 of the cathode 2 is heated by a beam 6 which is generated in an auxiliary radiation source 8. In this preferred embodiment, the auxiliary radiation source is constructed as a laser, a beam in the infrared, visible or ultraviolet wavelength range of which is incident on the wire tip 5 via a window (not shown) which is adapted to the wavelength of the radiation and an opening 9 in the first anode 3. Via borings l0 and 11 in the first anode 3 and the second anode 4, respectively, a beam current to be used in the scanning electron microscope is separated from the overall emission current of the wire tip 5. Via a further opening and window (not shown), preferably arranged on the side of the laser beam, the wire tip can be observed, if desired. The energy current to be applied to the wire tip is controlled by means of a control mechanism l2.

The following parts of the scanning electron microscope are also shown: a condensor lens 14, a deflection unit 15, an objective lens 16, an object or specimen l7, and detectors l8 and 19. A signal which is received by one of the detectors is displayed, via a signal amplifier 20, on a television monitor 21 which is coupled to the deflection unit 15.

During operation, the first anode has a potential of,

for example, a few thousands of volts with respect to the wire tip so as to realize the conditions for temperature field emission. The second anode then has a potential of many thousands of volts positive with respect to the wire tip. These potential values obviously are codependent of the geometry of the two anodes, the dimensions of the borings in the anodes, the distance between the anodes, and the distance between the anodes and the wire tip.

As was already stated, a field strength of approximately 10 to l KV/m must be generated on the surface of the wire tip so as to achieve the desired emission. Because the emissive tip is also formed by the field strength, a lateral displacement of the wire will not cause an inclined source position, provided it remains within given limits.

FIG. 2 is a diagrammatic representation of an arrangement for adjusting the wire cathode to the desired emission properties. A laser beam 30, originating from a continuously radiating laser 31, is converged at 'the area of the wire tip 5, by way of a lens or lens system 32, so as to form a focal spot having a very small transverse dimension which is determined exclusively by deflection phenomena of the coherent laser radiation.

The cathode wire 2 is held in a clamping device which is incorporated in the holder 1 and which will be described in detail with reference to FIG. 3. The clamping device serves to hold the wire tip of the wire cathode 2 accurately in a fixed position during operation of the electron beam apparatus, and hence to compensate for the evaporation thereof by supplying cathode material. To this end, the first anode 3 is connected to a direct voltage source 33. Due to the emission current, a voltage difference arises across a resistor 34; this difference is compared, by way of a direct voltage amplifier 35, with a voltage originating from a source 36. -A difference signal thus obtained is integrated in an integrator 37 and controls, via a current amplifier 38, a transport mechanism of the clamping device. By means of this transport mechanism the cathode wire 2 can be slowly displaced in the wire direction, for example, a few microns per minute. The wire tip 5 then more or less projects into the laser beam, so that the energy supplied thereto is controlled. This adjusting mechanism makes readjustment for any slow variations in the laser intensity or the geometry of the wire tip, for example, caused by evaporation of the material.

For adjustment in the case of quick variations, a control signal is used which is derived from an anode current to be supplied by a high voltage source 39 via a resistor 40. Via a capacitor 41 and an amplifier 42, this signal controls an electrodynamic converter 12 which adjusts the position of a blade 43 which is arranged in the laser beam. Using this blade, part of the laser beam can be intercepted, with the result that the energy applied to the wire tip is reduced. The laser beam can then be constricted on one side, on more than one side, or all around.

FIG. 3 shows a preferred embodiment of a clamping device. A straightened tungsten wire 2 is held between jaws 50 and 51. A resilient element 52 which at the same time serves as a hinge for the jaw 51 delivers a clamping force. The jaws 50 and 51, the resilient element 52 and a platform 53 together constitute a grip which can be displaced, by means of a parallel spring system consisting of the springs 54, 55, 56 and 57, in only one direction, i.e., along the axis of the gun. The platform 53 is pulled to a mounting plate 59 by a wire 58. The platform 53, and hence the cathode wire 2, is displaced by thermal length variation of the wire 58. During this displacement, a second set of jaws 59, 60 is open. As soon as a stroke extremity of the platform 53 is reached, the jaws 59, 60 close about the cathode wire, the jaws 50, 51 open, and the platform returns to a starting position so that the described process can be repeated. In a further preferred embodiment (not shown) a coupling between the jaws prevents the two sets from being simultaneously open. This coupling can be rendered inactive for inserting a new cathode wire.

This wire 58 is heated during operation by a filament current which is supplied by the current amplifier 38 (see also FIG. 2). Because the clamping faces of the jaws 50, 51 and 59 60 are arranged to be perpendicular with respect to each other, a newly inserted cathode wire 2 will be fixed along the line of intersection of the two clamping faces after a few take-overs. As a result, this mechanism automatically reproduces the location of the wire axis very accurately. The opening and closing of the jaws 50, 51 and 59, 60 for transporting the cathode wire is controlled by current control i.e., thermal length variation or wires 62 and 63.

Due to the use of a thin wire and the continuous very accurate readjustment of the wire tip, an emission current density occurs notably in the wire direction which is much higher than in known thermal electron sources of this kind, whilst the drawback of insufficient effective current of the field emission source, due to the limited dimension thereof, is at the same time overcome. it was found that, due to evaporation and surface migration, a wire tip having a curvature radius of approximately 1 micron is maintained on the continuously advancing cathode wire. As a result, the said Schottky effect or the so-termed temperature-field emission can occur in the strong electric field at the area of the wire tip at the high temperature ofthe tip at which it obtains the desired dimension. A tungsten wire having a diameter of 10 microns used as the cathode wire thus produces an emission current density in the order of 10 A/cm at a temperature of approximately 3,500 K from a wire tip having a curvature radius of approximately 1 micron which is arranged opposite to an anode which has a voltage of +2,000 V with respect to the wire tip. At the given transverse dimension an electron beam can be readily separated from a source having such a high current density, the said beam being particularly suitable for use as the beam current in a scanning electron microscope employing television techniques for displaying the image information.

An electron source according to the invention cannot only be successfully used in the described scanning electron microscope, but also, for example, in a transmission electron microscope, an electron beam machining apparatus, and a microanalyzer.

What is claimed is:

1. An electron beam source, comprising:

a cathode wire having a free tip;

means for generating an electric field at said tip of approximately 10 to 10 KV/m for drawing electrons away from said tip;

means for heating said tip to a temperature where deformation of said tip occurs due to said electric field;

means for forming an electron beam from electrons drawn away from said tip; and

means for supporting said cathode wire and for slowly advancing said wire in the axial direction thereof in accordance with the evaporation rate of said tip to maintain said tip in substantially the same position,

whereby said tip is maintained substantially point-like with a substantially constant radius of curvature of approximately 0.2 to 2 microns.

2. An electron beam source, comprising:

a cathode wire having a free tip;

an anode positioned adjacent said tip, said anode having an aperture for forming a beam from electrons emitted from said tip; means for supplying to said anode a voltage sufficiently positive with respect to said tip to generate a field strength at said tip of approximately to 10 KV/m;

an auxiliary radiation source positioned to focus radiation on said tip to heat said tip to a temperature where deformation of said tip occurs due to said field strength at said tip;

means for supporting said cathode wire and for slowly advancing said wire in the axial direction thereof in accordance with the evaporation rate of said tip to maintain said tip in substantially the same position, whereby said tip is maintained substantially point-like with a substantially constant radius of curvature of approximately 0.2 to 2 microns.

3. An electron beam source as defined in claim 2 wherein said aperture of said anode is substantially aligned with the axis of said cathode wire.

4. An electron beam source as defined in claim 3 wherein said means for supporting and advancing comprises means for controlling the position of said wire in accordance with the anode current of said anode in a direction which tends to maintain said anode current constant.

5. An electron beam source as defined in claim 4 wherein said cathode wire has a diameter of approximately 10 to 30 microns.

6. An electron beam source as defined in claim 4 wherein said auxiliary radiation source comprises a continuous laser.

7. An electron beam source as defined in claim 4 and further comprising a second anode cooperating with said anode position adjacent said tip in forming a beam, said second anode having an aperture aligned with said aperture of said anode positioned adjacent said tip and aligned with said tip.

8. An electron beam source as defined in claim 7 and further comprising means for controlling the intensity of radiation focused on said tip in accordance with the anode current of said second anode in a direction which tends to maintain a constant said anode current of said second anode.

9. An electron beam source as defined in claim 8 wherein said cathode wire is a substantially straight tungsten wire, the tip thereof being heated by said auxiliary radiation source to a temperature of approximately 3,500K.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. I 3 864 572 DATED February 4, 1975 INVENTOR(S) KAREL DIEDERICK VAN DER MAST ET AL I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On'the title page, after Section [2].] insert the following:

-:[30] Foreign Application Priority Data Oct. 3, 1972 Netherlands ..7213355-- Column 4, line 10, after "59" there should be a line 18, "or" should be of I Claim 7 line 3, "position" should be -positioned- Signed and Sealed this Fifteenth D y f March 1977 [SEAL] A ttes t:

RUT" C. MASQN C. MARSHALL DANN 17 Commissioner all-areal: and Trademark: 

1. An electron beam source, comprising: a cathode wire having a free tip; means for generating an electric field at said tip of approximately 105 to 106 KV/m for drawing electrons away from said tip; means for heating said tip to a temperature where deformation of said tip occurs due to said electric field; means for forming an electron beam from electrons drawn away from said tip; and means for supporting said cathode wire and for slowly advancing said wire in the axial direction thereof in accordance with the evaporation rate of said tip to maintain said tip in substantially the same position, whereby said tip is maintained substantially point-like with a substantially constant radius of curvature of approximately 0.2 to 2 microns.
 2. An electron beam source, comprising: a cathode wire having a free tip; an anode positioned adjacent said tip, said anode having an aperture for forming a beam from electrons emitted from said tip; means for supplying to said anode a voltage sufficiently positive with respect to said tip to generate a field strength at said tip of approximately 105 to 106 KV/m; an auxiliary radiation source positioned to focus radiation on said tip to heat said tip to a temperature where deformation of said tip occurs due to said field strength at said tip; means for supporting said cathode wire and for slowly advancing said wire in the axial direction thereof in accordance with the evaporation rate of said tip to maintain said tip in substantially the same position, wHereby said tip is maintained substantially point-like with a substantially constant radius of curvature of approximately 0.2 to 2 microns.
 3. An electron beam source as defined in claim 2 wherein said aperture of said anode is substantially aligned with the axis of said cathode wire.
 4. An electron beam source as defined in claim 3 wherein said means for supporting and advancing comprises means for controlling the position of said wire in accordance with the anode current of said anode in a direction which tends to maintain said anode current constant.
 5. An electron beam source as defined in claim 4 wherein said cathode wire has a diameter of approximately 10 to 30 microns.
 6. An electron beam source as defined in claim 4 wherein said auxiliary radiation source comprises a continuous laser.
 7. An electron beam source as defined in claim 4 and further comprising a second anode cooperating with said anode position adjacent said tip in forming a beam, said second anode having an aperture aligned with said aperture of said anode positioned adjacent said tip and aligned with said tip.
 8. An electron beam source as defined in claim 7 and further comprising means for controlling the intensity of radiation focused on said tip in accordance with the anode current of said second anode in a direction which tends to maintain a constant said anode current of said second anode.
 9. An electron beam source as defined in claim 8 wherein said cathode wire is a substantially straight tungsten wire, the tip thereof being heated by said auxiliary radiation source to a temperature of approximately 3,500*K. 